1. Field of the Invention
This invention relates to magnetic recording mediums such as, for example, magnetic tapes.
2. Description of the Prior Art
For high density recording, it is usual to use very finely divided magnetic powder in a magnetic layer of magnetic recording mediums. The use of the very finely divided powder results in a large light transmittance of the magnetic layer, so that magnetic recording and reproducing apparatus may not be operated normally. In domestic video tape recorders, for instance, the magnetic tape being run is stopped on detection of a transparent leader tape portion which is provided at one end of the magnetic tape. More specifically, the apparatus includes a tape end detector which is able to detect light which is transmitted through the leader tape portion, by which the magnetic tape is stopped. When the light transmittance becomes large in portions other than the leader tape portion, the tape end detector is liable to malfunction.
In order to prevent the malfunction of the tape end detector, carbon black is generally added to the magnetic layer in large amounts so as to reduce the light transmittance of the magnetic layer. However, when carbon black is added to a magnetic paint in large amounts, dispersability of a magnetic powder becomes poor, bringing about a lowering of electromagnetic conversion characteristics of the resultant magnetic recording medium.
In order that the light transmittance in portions of the magnetic layer is reduced without addition of large amount of carbon black to the magnetic layer, there has been proposed a magnetic recording medium in which a back coat layer containing carbon black is formed on a support on the side which is opposite to a magnetic layer-bearing side.
For the formation of the back coat layer, it is necessary from the standpoint of productivity to prepare a paint containing large amounts of carbon black. However, the carbon black-containing paint tends to harden or coagulate within a short time. The pot life of the paint is short because of the presence of carbon black. Accordingly, the prepared paint has to be applied onto a support before commencement of the hardening, thus impeding the productivity of the medium to a substantial extent. If, on the contrary, the content of carbon black is reduced so as to prolong the pot life, a satisfactory light transmittance cannot be expected in spite of the formation of the back coat layer.